My invention comprises improvements in an electronically controlled pressure regulator for use in an automatic transmission control circuit. A typical automatic transmission that may incorporate the improvements of my invention may be seen by referring to U.S. Pat. No. 4,351,206, which is assigned to the assignee of this invention. That patent describes a multiple ratio planetary transmission mechanism having clutches and brakes for controlling the relative motion of planetary gear elements to establish multiple forward driving ratios and a single reverse ratio. It discloses also an engine driven pump which is used to supply control pressure for an automatic control circuit. The circuit includes fluid pressure operated clutch servos and fluid pressure operated brake servos which are selectively controlled by the control valve circuit. Pressure is regulated in the control valve circuit by a pressure regulator valve that receives pressure signals that are proportional in magnitude to vehicle speed as well as to engine torque to establish a variable circuit pressure, the magnitude of which is controlled to provide the maximum pressure necessary to maintain the desired torque capacity of the clutches and brakes under varying driving conditions.
The regulator valve described in U.S. Pat. No. 4,351,206 is actuated by a control solenoid. The solenoid establishes a valve actuating force and the magnitude of the force of the solenoid depends upon a voltage signal that is proportional to engine torque. Engine torque is measured by the vehicle engine throttle position. An appropriate displacement-to-voltage transducer is provided for this purpose.
The regulator valve of U.S. Pat. No. 4,351,206 receives pressure directly from the control pump that is driven by the engine. Thus the valve is subjected to relatively high pressures.
Another example of a related control circuit is described in U.S. Pat. No. 3,727,487. That circuit includes a pressure regulator valve for an automatic transmission wherein the valve is subjected to a pilot pressure developed by a pilot valve that is under the control of a variable orifice solenoid. The solenoid is actuated by an engine torque signal. The engine torque signal is developed by a transmitter that responds to vehicle engine accelerator position. That signal is amplified and distributed to the variable force solenoid. The output pressure of the pilot valve is distributed to the main regulator valve. The pilot valve also is subjected to the pressure made available to the main regulator valve. Both the main regulator valve and the pilot valve are supplied with pressure by the engine driven pump.
U.S. Pat. No. 4,293,002 shows still another example of a valve circuit that includes a variable force solenoid to develop an optimum circuit pressure in an automatic transmission control. That patent describes a pilot valve that receives pressure through passages that are subjected to line pressure, and in this respect the teachings of U.S. Pat. No. 4,293,002 are similar to the teachings of U.S. Pat. No. 3,727,487 described above.
The solenoid efforts in these prior art circuits are undesirably high because they are subjected to full line pressure. Also, these prior art circuits require a constant supply pressure and the pressure drop that must be developed because of this is a variable pressure drop at all levels of pressure output. This is not conducive to an accurate, predictable relationship between pressure and current.